A serious environmental problem worldwide is the contamination of particulate materials, such as soils, sediments, fly ash, carbon and sludges, with toxic substances and/or radioactive substances, particularly from industrial applications. It is not uncommon to find particulate materials that are contaminated with numerous heavy metals and are also radioactively contaminated with radionuclides. There are known health risks to the public and to the environment that are associated with various heavy metals, including but not limited to mercury.
A thermal retort system, followed by stabilization of the recovered mercury, is the U.S. Environmental Protection Agency's Best Demonstrable Available Technology for treating contaminated particulate materials having greater than 260 ppm of mercury. A thermal retort system is typically comprised of a vacuum heating unit and distillation/condensation systems to recover metallic mercury from salts, solid objects, soils and coproducts from other facility processes. An example of a thermal retort system is shown in U.S. Environmental Protection Agency Demonstration Bulletin, EPA/540/MR-02/078, November 1992, which is incorporated by reference herein. These devices produce a semi-pure metallic mercury liquid which is resold directly or sent to a triple distillation process for further purification. The triple distillation process uses a vacuum distillation system to remove traces of impurities from metallic mercury so it can be resold as pure “technical grade” mercury. In cases where radioactive contaminated solids are treated, recovered mercury must be amalgamated prior to disposal as described in 40 C.F.R. § 268.42 Table 1. Thermal retorts, though effective, have drawbacks, namely a low throughput of waste handling capacity, a high capital and operating cost, and the potential for harmful off gases which must also be treated. Given that the mercury is heated to vaporization and then condensed, the possibility for mercury to escape or emit in an off gas from a thermal retort process is high.
Due to the high costs and emission problems associated with thermal retorts, other non-thermal (chemical) techniques have been attempted, with varying degrees of success, for stabilizing mercury and other heavy metals in particulate materials. For example, powdered zinc has been used to collect liquid elemental mercury in soils. Once the mercury is collected on the metal, liquid sulfur (polysulfide) is used to react with the mercury to form mercuric sulfide. However, zinc is an environmentally regulated metal as it maybe leached from the waste during the Toxic Characteristic Leaching Procedure (TCLP). Moreover, the prior art is focused on stabilizing mercury in aqueous waste streams. These treatments do not address the treatment of particulate materials contaminated with mercury, and may not be effective in treating particulate materials for many reasons. For example, the difference in the properties of an aqueous stream are vastly different from those of a solid or particulate material.
Another chemical method used to stabilize liquid elemental mercury was described in U.S. patent application Ser. No. 09/258,659 (about to issue) for stabilizing waste liquid elemental mercury, incorporated by reference herein. In this method, a combination of powdered sulfur and liquid sulfur (polysulfide) were used to react with the elemental mercury to form black mercuric sulfide (meta-cinnabar). However, merely placing elemental mercury in a bed of sulfur or in a volume of polysulfide does not yield the desired mercuric sulfide reaction. Sufficient heat must be available to overcome the energy of activation for the reaction to occur. In this method, intense mixing provided the necessary energy to initiate the reaction. Sufficient heat was generated from the reaction to escalate the rate at which mercuric sulfide was formed. In some instances, it was necessary to add water to the mixture in order to cool the reactants and control the reaction.
Attempts to use chemical methods on soils and sludges spiked with elemental mercury have largely been unsuccessful. These methods were generally ineffective in stabilizing mercury for two reasons. First, heat provided by mixing action or generated from the mercury-sulfur reaction quickly dissipates throughout the surrounding matrix materials, so it is difficult to overcome the energy of activation and initiate the mercury-sulfur reaction not just locally but throughout the entire reaction mass. Second, mercury in soils is not a continuous phase, as is the case when treating liquid elemental mercury. Chemical stabilization of mercury and other heavy metals in solid materials is difficult because of the dispersed nature of the contaminant within the solid matrix and the consequent problem of contacting a stabilizing chemical additive(s) with such a widely dispersed contaminant. Distance between the reaction sites is great enough to prevent the reaction from propagating from one site to another.